Various mechanical, electromechanical and sonic techniques have been employed to measure the level of liquid in a container or the interface levels between two or more liquids. The mechanical and electromnechanical systems are relatively slow in reacting to changes in the level of the liquid, while systems employing sonic or ultrasonic transmissions become complex when used to measure the surface levels of contained liquids because of the echoes received from the surfaces forming the sides of the container.
Most prior art radiation reflection detection systems, while providing highly accurate indications of liquid level and responding in extremely short times to changes in the levels of the contained liquids, require very complex and expensive apparatus in order to function properly. One prior art system which attempts to minimize this complexity is shown in the U.S. Pat. No. 3,832,900. This system utilizes an open coaxial line which is immersed in the contained liquid, the contained liquid thereby filling the coaxial line. The liquid surface creates a discontinuity in the coaxial line which produces a reflection of the base band pulse signal that propagates back along the transmission line. The time at which this reflection is received, relative to the time of the transmitted pulse, determines the level of the liquid. However, the transmission line tends to clog and requires frequent cleaning.
Another apparatus for measuring the level and dielectric constant of liquid is shown in the U.S. Pat. No. 3,995,212. This device generates a subnanosecond base band pulse-that propagates through a transition device to a single wire transmission line that extends through an air filled region into the liquid. The discontinuity created at the air-liquid interface produces a reflection of the base band pulse that is compared with the transmitted pulse with respect to time of arrival and amplitude to determine the liquid level and the reflection coefficient at the air liquid interface. The reflection coefficient is then utilized to determine the dielectric constant of the liquid.
Another fluid level sensor is shown in the U.S. Pat. No. 5,457,990 wherein a fuel level sensor includes a coaxial conductor probe that is positioned within a fuel tank at an angle relative to vertical and horizontal references. A repetitive electrical signal having a train of transients is transmitted from the upper end of the line and the line is monitored for reflections exceeding a threshold to generate an analysis window during which time the received reflected signals are analyzed to determine the level of the liquid in the tank.
The U.S. Pat. No. 3,812,422 discloses another time domain reflectometry fuel measuring device. This fuel measuring device requires a coaxial arrangement that has a grounding device between the inner and outer conductors. The grounding device is preferably in the form of a metallic plug at the bottom of the probe between the two conductors without which accurate measurements cannot be acquired.
A micropower impulse radar studfinder device utilizing time domain reflectometry is shown in the U.S. Pat. No. 5,457,394. Unmodulated pulses, approximately 200 ps wide, are emitted from a generator positioned on the surface of a wall. A large number of reflected pulses are sampled and averaged while background reflections are subtracted. Reflections from wall studs or other hidden objects are detected and displayed using light emitting diodes.
A microwave device for measuring the level of liquid in a container is shown in the U.S. Pat. No. 4,359,902. A "magic tee" has an "H" arm connected to a microwave sweep oscillator generating a sweeping frequency microwave signal, a first collinear arm connected to a termination, a second collinear arm connected to a waveguide inserted into a container of liquid and an "E" arm connected to an "RF" input of a mixer. A "LO" input of the mixer is connected through a coupler to the oscillator output and an "IF" output of the mixer is connected to a detector for sensing a frequency difference between a reflected signal and another output signal of the oscillator.